Enable software decoding video with libvpx on Android
Enable software decoding video with libvpx on Android

VpxVideoDecoder previously would reject VP8 without an alpha
channel, leaving it to be decoded by FFmpegVideoDecoder. Now,
when it's compiled without FFmpegVideoDecoder it will handle
all VP8.

BUG=507834

Review URL: https://codereview.chromium.org/1281433002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#342405}

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